Why is my pullet hanging out by herself?

I have a group of four 19ish week old chickens (I say "ish" b/c I'm not exactly sure how old they are, but most people here guessed that they were 10 weeks old on the 1st of June, so that would put them at 19 weeks), one pullet and 3 cockerels. I started with six, but two have already been butchered. One more is slated for butchering, but we are waiting for him to get a bit bigger b/c the last one was on the small size. I have 10 more younger chickens and 25 more coming next week. The four have been together since I got them around 6 weeks old.

Anyway. For the past two days, my oldest pullet has been hanging out by herself, away from the cockerels in her group. As far as I can tell, they aren't being aggressive to her or making her stay there. She stands for hours under a bush on the corner of our house, or she is in the coop. Yesterday the head cockerel came over and chased her out of the bush a couple times. This evening she was hanging out with the boys again with no problem.

She hasn't started laying, so I thought maybe that was what she was doing, but I keep checking the bush and the coop and no egg yet.